Doppler wind lidar using a MOPA semiconductor laser at stable single-frequency operation

Lasers of ultranarrow linewidth, stable single frequency, exceptional beam quality, and high power across versatile wavelengths in the visible spectrum are essential for a variety of applications including the generation of artificial guide stars, atom trapping in quantum experiments, and optical lattices for optical clocks. Diamond Raman lasers emerge as a promising solution since they can access wavelengths in the spectrum that no other laser can reach. They enable power scaling without compromising beam quality due to the exceptional diamond's thermal conductivity. Notably, Raman lasers offer a phase-noise-cleaning mechanism that is orders of magnitude superior to the traditional Brillouin lasers [1]. They inherently exhibit a spatial-hole-burning-free nature enabling the single-frequency laser operation [2]. However, achieving long-term stable single-frequency operation remains a significant challenge. To date, our group and others have demonstrated only short-term single-frequency operation, typically lasting from seconds to several minutes at power outputs in the few-watt range [3], [4]. This has been attributed to the higher order spatial stimulated Brillouin scattering (SBS) which causes chaotic high-frequency modes and also to the relatively broad Raman gain spectrum compared to the laser cavity's free spectral range which leads to multimode operation by the thermal effects at high power levels. In this contribution, we present our latest advancements in the development of high-power diamond Raman laser with exceptional spectral quality and stable single-mode operation in the visible.

In this paper we present an experimental analysis of the effect of RF modulation injection current in DFB and FP lasers nearthe first period doubling bifurcation (PDB). By increasing the amplitude of the modulation frequency at the onset of PDB thefirst results of a virtual Hopf phenomenon like behavior has been observed in both types of lasers. We characterized thisobservation by increasing the amplitude of the current modulation where two peaks appear in the spectrum, approaching oneto the other until fmally merging the onset of period doubling bifurcation as predicted by theory. The same virtual Hopfbehavior was observed when we set the amplitude modulation at a fixed value and we sweep the frequency modulation at theonset of the PDB point.Keywords: Laser Diode, Nonlinear Dynamics, Period Doubling Bifurcations. 1. INTRODUCTION Nonlinear dynamics in diode lasers has been the subject of many theoretical and experimental investigations in the lastdecade as they show a rich variety of irregular behavior in their route to chaos1. A single frequency diode laser is commonlystudied by a rate equation model which describes the time evolution of the photon and carrier densities. In these equations,the inclusion of an additional degree of freedom as modulation of the injection current, external light injection or opticalfeedback is needed for nonlinear dynamics to appear2.To our knowledge, the nonlinear behavior in semiconductor diode lasers has been extensively studied by many authors usingdelayed optical feedback and strong current modulation. In the former, the observed route to chaos was through successivesubharmonic bifurcations or by quasi periodic route3'4. In the strong current modulation case the period sequence wastruncated by period tripling before the onset of chaos5'6.On the other hand, the onset of a period doubling bifurcation point in many physical systems brings to nonlinear phenomenasuch as: 1) small signal amplification, 2) shift and suppression of the bifurcation point and 3) the emergence of a closelyspaced set of peaks in the frequency response7.In semiconductor lasers the parametric amplification was observed by Harth and co-workers8'9 which was analyzed byWiesenfeld few years after'°. In the study developed by Wiesenfeld in ref. 10, semiconductor lasers were proposed as goodcandidate to study the rich variety of nonlinear phenomena near period doubling bifurcations. A recent study has shown thatthe effect of Langevin noise in period doubling semiconductor lasers has a virtual Hopf ur12 previously predicted byWiesenfeld'34 as a virtual Hopf phenomenon in systems undergoing a period doubling bifurcations in the presence ofexternal noise.In the present study we have experimentally investigated the nonlinear dynamics of diode lasers in the period doublingbifurcation point. We study the influence of the amplitude modulation in Distributed Feedback (DFB) lasers near perioddoubling bifurcations. In Fabry-Perot (FP) lasers we performed as well the same study as DFB lasers and the analysis of thefrequency modulation near period doubling bifurcation. So in section II we described the experimental procedure for theonset of the first period doubling bifurcation performed in commercial DFB and FP InGaAsP diode lasers at 1,3 tm emission

Distributed feedback (DFB) lasers are used extensively as optical sources within fiber optic telecommunications systems. These semiconductor lasers are valued for their single optical mode operation which enables their use in high bit rate digital and broadband analog transmission systems. We discuss requirements imposed upon DFB lasers used in such applications. We briefly review and illustrate general techniques for modeling DFB laser operation by comparison of measured and modeled device characteristics below and above lasing threshold. We focus on the phenomenon of spatial hole burning which substantially affects operation of all DFB lasers, especially those tailored to produce high fiber coupled power from a single mirror facet. We illustrate insights gained by measurement and modeling of DFB lasers using as an example the particular case of analog transmission of multichannel video signals.

Tunable semiconductor laser has various applications including Wavelength Division Multiplexing (WDM), Frequency Division Multiplexing (FDM), optical switching in Local Area Networks (LAN) and Chemical Sensing, or Spectroscopy. There are different approa ches for tunable semiconductor lasers. Large wavelength tuning range is required for communication applications, such as WDM. Whereas a frequency modulation (FM) requires small but fast frequency shifts. Two-Segmen t laser is one of the methods to realize continuous optical tuning. The device consists of two coupled cav ities having a series of well-defined modes. The modes are equally spaced within a cavity but the mode spacing between the two cavities is slightly different. Characterization results of such lasers are very important. In this paper we will present the characterization results of two segment InGaAs/InP distributed feed back (DFB) tunable lasers. Keywords: DFB Lasers, tunable laser, two segment laser, semiconductor lasers. 1.0 Introduction

The implementation of state-of-the-art semiconductor lasers with distributed feedback (DFB), and the optimization of the DFB resonator for specific operating characteristics, require, in many cases, a grating with carefully tailored, slowly varying parameters along the cavity axis. A change in the corrugation pitch and incorporation of discrete phase shifts along the cavity may be implemented by E-beam lithographic methods, and by changes in the geometrical parameters of the laser waveguide. A change in the coupling strength, |/spl kappa/|, is more difficult to realize. Recently, a novel method for fabricating a DFB laser with two discrete values of |/spl kappa/| in different sections of the laser cavity, was demonstrated, but the extension of this method to arbitrary variations in |/spl kappa/| along the laser is not straightforward. Selective organometallic vapor phase epitaxy (OMVPE) on masked substrates is an attractive option for epitaxial layer deposition with controllable thickness variations. Both the growth rate and the composition of ternary or quaternary layers are determined by the mask geometry. In this work, a novel method for the formation of DFB gratings is demonstrated, relying on selective OMVPE growth. The method does not require semiconductor etching, and furthermore it allows for a controlled variation of |/spl kappa/| along the laser cavity. >

External optical feedback sensitivity in distributed feedback (DFB) semiconductor lasers is analyzed in this paper with special attention to phase-shifted and complex-coupled lasers. The effects of variations of structural parameters such as index and/or gain coupling strengths, facet reflectivities, and corrugation phase angles on external optical feedback sensitivity are studied. Quarter-wave-shifted index-coupled DFB lasers exhibit low external optical feedback sensitivity for large index coupling coefficients and high facet reflectivities. Pure gain-coupled DFB lasers perform better than the uniform index-coupled DFB lasers without any phase-shifting structure but worse than /spl lambda//4 phase-shifted index-coupled lasers with high coupling strengths. External optical feedback sensitivity of complex-coupled lasers is a sensitive function of the structural parameters and varies significantly with the index-to-gain coupling ratio and the total coupling. Presence of a small amount of index coupling relative to the gain coupling in a gain-coupled laser improves the external optical feedback sensitivity.

In this paper, we present the theoretical investigation of index-coupled distributed feedback (DFB) laser with tilted single mode ridge waveguides. By tilting part of the ridge waveguide in various degrees, DFB laser with manifold effective grating periods can be realized. The structure is analyzed using couple mode theory in matrix form based on threshold analysis. Important parameters of DFB laser like resonant frequency and threshold gains are obtained by solving the eigen-equation. The results indicate not only that the lasing frequency is modulated by the waveguide titling angle, but also large Gain Margin (GM) can be achieved at the threshold condition which enhance the stable single mode operation in index-coupled DFB laser.

Summary form only given. Numerous applications such as free space optical communications, metrology and differential absorption lidar systems require high brightness laser sources with spectral purity. Monolithically integrated Master Oscillator Power Amplifiers (MOPAs) are promising candidates to fulfil these requirements. In particular, a three-section bent mOpA at 1.5 μm was recently proposed as laser source for an integrated path differential absorption lidar system for measurement of atmospheric CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> [1]. The device consists of a distributed feedback (DFB) section acting as master oscillator, a bent modulator (MOD) section and a tapered semiconductor optical amplifier (SOA) section with a tilted front facet to avoid coupled cavity effects [2]. Three separate electrical contacts on the laser chip provide access to the three sections: the DFB current controls the laser emission frequency, the MOD current allows for modulation and the SOA current drives the amplification. The MOD section acts as an absorber or amplifier when driven at zero or positive voltage bias respectively. Complete output extinction requires negative values of the MOD current due to carrier generation caused by the power injected from the DFB into the MOD section [1].Here, we theoretically investigate the observed characteristics of a three-section MOPA both in CW and under modulation operation by means of a Traveling Wave Model (TWM) [3] that naturally includes spatial effects (such as spatial hole burning and coupled-cavity effects) and multimode dynamics. The multi-section TWM [4] has been modified in order to reproduce the electrical behaviour of the MOPA. Specifically, we consider the voltage applied to each section as an input parameter, and then the local current density depends on space through the local carrier density, and it can be positive when the modulator is acting as an amplifier or negative when the modulator is absorbing the light injected by the DFB section.

Single-longitudinal mode Nd:YVO4/YVO4/KTP green solid state laser

The operating characteristics of distributed feedback (DFB) InGaAsP lasers are reviewed from the point of view of their applications in optical communication systems. The wavelengths and the threshold gains of the allowed longitudinal modes of a DFB laser are obtained using the coupled-wave theory after incorporating both the facet reflections and the grating-induced distributed feedback. The experimental results are presented for 1.55 μm DFB semiconductor lasers. In particular, we discuss the temperature dependence of the threshold current and the emission wavelength, the modulation response, the frequency chirp, and the linewidth of the single longitudinal mode selected by the DFB mechanism.

This paper emphasizes methodology concerning variations of the emission wavelengths of edge-emitting semiconductor lasers. We present a method based on tilted distributed feedback (DFB) gratings to be used in combination with specially bent or tilted straight waveguides for the low-cost and ultraprecise definition of different wavelength channels in lasers for Dense wavelength division multiplexing (DWDM) fiberoptic communication systems. The homogeneous DFB grating field is defined using low-cost processes and is tilted with respect to a preferential direction defined by the device borders or the crystal geometry. Our method allows ultraprecise definitions of the DWDM wavelengths and is applicable index- and complex-coupled DFB gratings in both isolated lasers and arrays. Design, technological implementation, and experimental characterization of the devices is presented in this paper. Thin-film heaters are used for correcting small wavelength deviations occurring due to technologically induced tolerances.

This paper presents a systematic investigation of the third-order intermodulation distortion characteristics in distributed feedback (DFB) semiconductor lasers. The influence of several nonlinearities, such as longitudinal spatial hole burning, gain compression, and relaxation oscillation, is considered. Detailed analysis shows that it is possible to make different nonlinearities cancel one another to give a low intermodulation distortion by choosing the appropriate DFB structure and beat conditions. Specifically, conditions for cancellation between spatial hole burning and gain compression nonlinearities are introduced.

A circuit model for absorption grating (AG) gain-coupled (GC) distributed feedback (DFB) semiconductor laser diodes (LD) is presented, based on the rate equations of carrier densities in active region and absorption grating, as well as the photon density. The relation of photon lifetime to cavity loss and facet loss, and the relation of light power to average photon density are derived. As an example, self-pulsation in AG–GC–DFB–LD is studied.

A frequency tunable optoelectronic oscillator based on a directly modulated distributed-feedback (DFB) semiconductor laser under optical injection is proposed and experimentally demonstrated. Through optical injection, the relaxation oscillation frequency of the DFB laser is enhanced and its high modulation efficiency can enable the loop oscillation with a RF threshold gain of less than 20 dB. The DFB laser is a commercial semiconductor laser with a package of 10 GHz, and its packaging limitation can be overcome by optical injection. In our scheme, neither a high-speed external modulator nor an electrical bandpass filter is required, making the system simple and low-cost. Microwave signals with a frequency tuning range from 5.98 to 15.22 GHz are generated by adjusting the injection ratio and frequency detuning between the master and slave lasers. The phase noise of the generated 9.75 GHz microwave signal is measured to be -104.8 dBc/Hz @ 10 kHz frequency offset.

We proposed a new control method of the tunable distributed feedback (DFB) laser array (TLA) to realize high-speed optical switching by changing the injection current without sacrificing reliability. Previous studies of high-speed wavelength switching at DFB lasers achieved a sub-second switching time by injecting a large current to the active layer. However, a large injection current caused laser degradation that seriously reduced laser lifetime and reliability. In this paper, based on fast wavelength switching by injection current control, a new switching method was proposed that reduces an averaged injection current by increasing temperature. We successfully demonstrated 400-GHz-wide wavelength switching, which is the maximum tuning range of a single DFB laser in the TLA, with a switching time of 130 ms and reduced the injection current from 402 to 52 mA. Thus, we were able to greatly improve the reliability and increase the lifetime of the TLA by 60 times.